The present invention relates to electrochemical cells. More particularly, the present invention relates to electrochemical cells using electrolytes, which are preferably capable of operating from ambient or low temperatures to higher temperatures such as about 170° C.
The concept of the lithium-ion or rocking chair batteries, where lithium ions intercalate and deintercalate between the cathode and the anode was introduced in the early nineteen eighties. In 1991, Sony Corporation introduced the first commercially viable lithium-ion secondary cell into the market. It contained coke as the anode and LiCoO2 as the cathode. Electrolytes used in commercial lithium-ion cells contain organic solvents such as ethylene carbonate, dimethyl carbonate, 1,2-dimethoxy ethane together with a lithium salt (e.g., LiPF6). During the first charge, solvent and the anion undergo reduction forming the solid electrolyte interphase (SEI); the lithium ion intercalation into the coke electrode occurs via the SEI. The SEI passivates the lithiated carbon anode from further reaction with the electrolyte and permits stable operation of the rechargeable cell. While this cell includes good charge and discharge cycling at ambient temperature and below, its high temperature operation is limited to 40 to 50° C. because of the volatility of the solvents utilized in making the battery, as well as the dissolution of the SEI in the electrolyte which leads to a thermal runaway.
The higher temperature performance of lithium-ion cells can be improved by incorporating anode materials which intercalate lithium ions at voltages higher than the reduction voltage of the electrolyte. Such cells show longer cycle life as solvent reduction at the electrode surface is eliminated, and do not suffer from a thermal runaway experienced by conventional lithium ion cells using coke or graphite as the anode. The higher voltage for lithium ion intercalation at the anode also eliminates the possibility of lithium metal deposition and dendrite formation, which shortens cell life. Possible anode materials for such cells include, Li4Ti5O12 (1.5 V vs. Li), LiWO2 (0.3-1.4 V), and LiMoO2 (0.8-1.4 V). In spite of the above described advantages, the use of traditional volatile solvents as electrolytes, still limits the higher temperature operation. Gel type solid polymer electrolytes can extend the higher temperature limit, but as the cell is heated, the liquid separates from the solid polymer, which is a limitation factor that affects high temperature operation. A solid (dry) polymer electrolyte has poor conductivity at ambient temperature; therefore, such an electrolyte permits cell operation only at higher temperatures. Lithium metal rechargeable cells are advantageous because of their potential for high energy densities. However, the use of organic solvents in the electrolyte limits high temperature operation due to solvent volatility. A lithium metal rechargeable cell developed by Tadiran Ltd. has a 125° C. upper operation temperature limit due to polymerization of its solvent, 1,3-dioxalane.
Another class of electrolytes that can be used in the lithium-ion cells is based on ionic liquids. Ionic liquids are molten salts that are liquids at temperatures below 100° C. Ionic liquids comprise entirely of ions (positive ions or cations and negative ions or anions). They, generally, have high ionic conductivity, high thermal stability and wide electrochemical windows. Further, unlike the solvents in standard lithium ion cells, the ionic liquids are non-volatile and non-flammable.
Low temperature molten salts can consist of mixtures of compounds, (i.e., anions and cations) which are liquid at temperatures below the individual melting points of each individual compound. These mixtures, commonly referred to as “melts,” can form molten compositions simultaneously upon contacting the components together or after heating and subsequent cooling.
Low temperature molten salts were used as electroplating baths by F. H. Harley and T. P. Wier, Jr. in 1948. These low temperature molten salts were obtained by combining aluminum chloride with certain alkylpyridinium halide salts, for example, N-ethylpyridinium bromide.
Since then additional ionic liquids were produced by mixing AlCl3 with different organic cations containing a variety of substituents; however, the field was dominated by those containing 1-ethyl-3-methylimidazolium cation. Although these ionic liquids were useful in studying electrochemistry of both inorganic and organic solutes as well as organic and organometallic reactions, the disadvantage of these ionic liquids was the presence of anions (AlCl4−, Al2Cl7−) derived from strong Lewis acid AlCl3 which liberate toxic gas when exposed to moisture.
In 1992, a new class of water and oxygen stable ionic liquids was described by Wilkes and Zaworotko, and by Cooper and Sullivan. These ionic liquids had the anion derived from the Lewis acid AlCl3 replaced by water and oxygen stable anions such as BF4−, CH3COO−, CF3SO3− and CH3SO3−. Some of these ionic liquids (e.g EMIBF4, EMITriflate) showed high electrochemical stability as well as thermal stability.
In addition, melts can contain an EMI+ cation and PF6− anion. Besides room temperature melts containing EMI+ cations, melts have also been prepared with different cations, such as the 1,3 dialkylimidazolium cation and the 1,2,3 trialkylimidazolium cation. For example, 1-(n-butyl)-3-methylimidazolium cation utilizing anions such as BF4−, PF6−, and AsF6− have been prepared. The latter melts show wider electrochemical windows than EMI+ containing melts; however, they also show lower conductivity and lower melting points. In addition, the above melts are not stable toward lithium, a strong reducing agent.
Another class of ionic liquids is based on pyrazolium cation which is a structural isomer of the imidazolium cation. Pyrazolium tetrafluoroborate ionic liquids were observed to be stable to lithium metal from room temperature to high temperatures (150 to 160° C.). The application of ionic liquids in lithium metal rechargeable cells from ambient temperature to higher temperature (130° C.) using pyrazolium cation based ionic liquids is described in U.S. Pat. No. 6,326,104 B1, which is incorporated in its entirety by reference herein. The electrolytes used in these cells are reduced by lithium metal anode forming a passivating layer on its surface. Increase in cell resistance and decrease in capacity with cycling observed in these cells may be due to breakdown and reformation of the passivating layer. In addition, the electrolytes used in these cells contained anions such as BF4− and AsF6−, that can dissociate to corresponding Lewis acids (BF3 and AsF5) and LiF, during high temperature operation, leading to cell deterioration. Further, U.S. Pat. No. 5,683,832 relates to hydrophobic liquid salts of imidazolium and Imide anions. U.S. Pat. No. 5,827,602 relates to specific hydrophobic ionic liquids and generally mentions various anions and cations, but only tests one of them and describes not preferred combinations to obtain electrical and physical properties critical to successful operation of the electrochemical cells.
The operation of passivation free lithium-ion cells, at ambient temperature, using ionic liquids is described in Nakagawa et al. (Yuasa-Jiho, 91, 31, 2001, J. Electrochem. Soc. 150, (6), A695-A700 (2003)) and Michot et al. (U.S. Pat. No. 6,365,301 B1). In one example, Li4Ti5O12/LiCoO2 electrodes containing 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4)/LiBF4 electrolyte was cycled at ambient temperature producing over 150 cycles. In another example, Li4Ti5O12/LiCoO2 containing 1-ethyl-3-methylimidazolium bis-fluorosulformimidide (EMIFSI)/LiFSI as electrolyte was used, where cycling results were not reported.
During the present studies it was observed that Li4Ti5O12/LiMn2O4 and Li4Ti5O12/LiCoO2 cells containing EMIBF4/LiBF4 or 1-ethyl-2-methylpyrazolium tetrafluoroborate (EMPBF4)/LiBF4 as electrolyte, operated with stable capacity at ambient temperature and at slightly higher temperature; however, capacity was rapidly lost with cycling at 80° to 100° C. (≈80% decrease in 20 cycles).
The above references provide no solution to providing an electrochemical cell that can operate over a large range of temperatures. Therefore there is a need for lithium-ion cells that will operate from low temperature to higher temperatures (e.g., 170° C.). Such cells will find applications in oil/gas drilling operations, and also in automotive, aircraft and space environment as well as other applications.